Many years ago, and it isn't hard. Cut the ends off the underframe casting, build new body bolsters, and body mount 1025 or 1015 couplers. I prefer 1025, but either will work. I make my bolsters from Evergreen 0.060x1/8 and 0.030x1/8 strip, thick glued to the floor, a piece of thin on that, drill a 5/64" hole for the pin. My trick was to use two pieces of the thick stuff, with a 3/32" gap in the middle, then drill the pin hole before gluing the thin piece on. No hole in the floor that way.
Add a new piece of center sill between the bolster and weight, as a view block, if desired.
The stirrups won't clear body-mounted couplers, so cut the center section out, glue the stirrups in place with something that will hold for a while, then glue styrene blocks to the inside of the end sills to lock the stirrup in place mechanically. If you're converting a drop-end gondola, the stirrups also hold the end in place, so they're needed. For a fixed-end car, one could also replace the stirrups with wire or etched parts.